Mediavine names 8,638 sites in its seller file against Primis’s 192, and 4 sites appear in both files. Reach is only half the question though — the authority spread below shows what kind of sites each one actually carries, which is what determines whether you would be accepted and what the inventory is worth.
Primis is the more selective of the two, with a median authority of 60 against 29. Read the overlap with care. Two display networks rarely run on the same site, so most of those 4 are publishers part-way through switching, still listed by the network they are leaving. A video network listed alongside a display network is the exception — that is usually a genuine pairing.
On 8,446 more sites than Primis — the larger footprint of the two.
Median authority of 29 against 60 — more of its sites are still small, so a growing site clears its bar sooner.
Mostly sites mid-switch between the two, unless one is a video network that runs alongside display. The “Also uses” column on either network page lists them.